This project comprises an ongoing 4-year prospective, double-bind, placebo- controlled clinical trial of calcium (Ca) supplementation in 236 elderly women who were 61 to 70 yrs of age at entry and at least 10 yrs postmenopausal. The clinical trial is unique as it is population-based, and the participants were obtained by screening this age-group in the entire population of Olmsted County, MN. At the time of submission, it was estimated that it would take up to 2 yrs to screen the 3,013 potentially eligible women and recruit participants into the trial and that an extension would be required to complete it. This has proved to be the case, and we are requesting a 1-yr extension. The central hypothesis upon which the trial is based is that a major etiologic factor for bone loss in elderly women is Ca supplementation. Our primary study objective is to determine the effect of Ca supplementation on bone loss, and our secondary study objectives are to determine if this is associated with a decrease in parathyroid function and in bone turnover and to determine side effects and patient acceptability of Ca supplementation in these elderly women. The participants were randomized into treatment and control limbs; the subjects in the treatment limb received 1,600 mg/day of Ca (as Ca citrate) and those in the control limb received a matching placebo. The subjects were evaluated at baseline and are seen every 6 months during treatment for assessment of therapeutic compliance and measurement of bone density (lumbar spine, proximal femur, and whole skeleton) by dual photon absorptiometry, biochemical markers for bone turnover (serum osteocalcin, bone alkaline phosphatase, and urinary pyridinium cross-links), and parathyroid gland function (serum intact parathyroid hormone and urinary cAMP). It is important to complete the full 4 yrs of the trial to have sufficient statistical power to resolve the central issue unequivocally and, because some studies have suggested a waning of the therapeutic effect after 1 yr, to establish that any beneficial effect is maintained long- term. Given the enormous magnitude of the problem of involutional osteoporosis, the only cost-effective approach is prevention. Intervention with Ca supplementation is an attractive option because, in contrast with other available drugs for the prevention of bone loss, it is relatively inexpensive and safe, and mass interventive strategies, such as Ca fortification of food, could be employed. However, before such nation-wide programs could be recommended, more definitive and rigorous data should be obtained supporting long-term efficacy and safety, such as those to be derived from this clinical trial.